Hey Dave,
I think this idea (in HILITE) is what Dr Gill was talking about as the "CSI effect" or confirmation bias, in that BBC Radio 4 interview.
Even if the bra could be shown to have never left the room, finding DNA on it isn't necessarily incriminating, anymore than finding Amanda's DNA in her own bathroom.
Even though bathroom DNA is a more likely find, the fact is Amanda lived there, and Raf had a legitimate reason to visit, means their DNA could show up, certainly in trace amounts, or even in reliable quantities.
Now Raf's DNA in Meredith's vagina, or Raf's sperm on the pillow beneath her, would be harder to explain.
But the point Dr Gill made is that DNA can identify people who have no plausible or legitimate reason to be in the house, like Rudy.
PLUS, DNA is looked at in combination with the physical context in which it is found.
Chris H has referred to Gill's description as source (sampled from visible stains), to subsource (LCN, just shows up in the results).
Raf's lawyer Bonjourno argued he was not a dragonfly that could zip into the room, participate in a bloody violent murder, not step in wet blood, touch nothing else, and leave a trace of DNA only on a tiny metal bra clasp and not the bra fabric as Guede did - along with 2-4 other guys who also could be presumed on the same evidence to have done the same things. (a feat Dr Mark Waterbury described as a "scientific impossibility").
AND, there is the fact that Amanda says they spent the night together at Raf's apartment, and there is witness and computer evidence to support that alibi.
So for me, I can't agree a simple finding of a trace amount of DNA could ever be conclusive evidence of presence in the room when the crime was committed.
Nor would a larger than trace sample provide a substantially stronger case, for the reasons you describe, and the surrounding circumstances I described.
In this case though, Stef antics in the lab make it fairly plain that none of these DNA results from the knife or bra clasp are even real and legitimate findings.
There is a reality, a truth at the bottom of all these discussions. And fiddling with minor ticks in evidence results, doesn't alter the underlying reality. One set of footsteps in Meredith's wet blood - start with that and try fitting anyone else in that room - I can't do it.
Just for fun, consider this scenario:
I saw a TV show where an angry woman in Montreal, seduced a neighbor, saved his sperm, and planted it on the body of another neighbor she had killed, because the other neighbor had killed her cat.
Imagine a real life jury dealing with that fictional case? Tough to defend? Under the view you've described, dude is toast.
